Radioactive isotopes emit unique spectra of radiation. The spectra are defined by nuclear transitions, and sometimes by subsequent atomic transition caused by the emission of a beta particle or other nuclear decay. As the nucleus transitions from an excited state to “ground state” energy can be given off in well defined pathways. Such pathways include, but are not limited to, gamma ray emissions, beta particles (electron and positron emissions), and alpha particle emissions. Each radioactive isotope has a unique spectra which can be referred to as the isotope's characteristic spectra.
The transmission of gamma rays and particles through matter is strongly determined by the energy of the gammas or particles and the material the gamma rays and particles travel through. The number of alpha particles and electrons will be attenuated very quickly as they travel through matter, and as alpha particles and electrons travel through matter they interact with the matter, causing some additional radiation to be emitted. The subsequently emitted radiation caused by the interaction of these initial particles with the matter in which the initial particles are traveling can radiate in all directions. Accordingly, the number of particles attenuates rapidly as the particles travel through materials. Gamma rays, which are very similar to very high energy X-rays, are much less likely to interact with matter as they pass through the matter, and, therefore, are not attenuated as rapidly as they pass through matter.
The type of material the radiation passes through also affects the transmission of the radiation. Higher atomic number materials attenuate the radiation more quickly. Lead attenuates most radiation very rapidly as it has very high atomic number and density.
Spectrometers are devices that are used to determine the energies of emitted radiation. Spectrometers typically work by using a crystal (such as NaI), solid state detector, or scintillator, to detect the radiation. The radiation interacts with the crystal (or scintillator) and produces light that is fed to a photomultiplier tube, which produces an electric pulse that is proportional to the energy of the original radiation. These electrical pulses are then processed by a multi-channel analyzer that commonly converts the signal into an energy spectrum. Spectrometers can be expensive and their accuracy depends on the quality of the radiation detector. Solid state detectors can be quite accurate detectors, but also can be quite expensive.